The present invention relates to a horizontal blanking pulse generation circuit used for a display unit having a cathode-ray tube.
A conventional horizontal blanking pulse generation circuit is disclosed in JP-A-57-183182. In this conventional circuit, a flyback pulse generated in a horizontal deflection output circuit is clipped at a predetermined potential thereby to form a horizontal blanking pulse.
The conventional television receivers feature what is called an "overscanning" in which the size of the raster scanned on the face plate of the CRT (Cathode-Ray Tube) is larger than the face plate of the CRT. In a display unit used for a computer terminal or the like, however, what is called an "underscanning" is a common phenomenon in which the size of the raster scanned is smaller than the face plate of the CRT. A display unit of this underscanning poses a problem described below.
The electron beam is reversed in scanning direction and therefore temporarily stops scanning at the ends of the raster scanned on the face plate of the display unit. Unless a blanking pulse is correctly formed, therefore, the brightness of the raster is extremely increased by the temporarily stopped electron beam at the ends of the raster scanned, and a bright spot is observed at the ends of the face plate. No bright spot would occur at the sides of the raster, if a blanking pulse is formed correctly. Actually, however, the delay time of a circuit for transmitting the horizontal blanking pulse causes a phase delay of the blanking pulse, with the result that the bright spot at the right edge of the raster is not easily erased.
The above-mentioned problem is specifically explained with reference to FIG. 8.
Waveforms of the voltage and current generated in the essential parts of a horizontal output circuit are shown in FIG. 8. As seen from FIG. 8(a), a saw-toothed wave current I.sub.t flows in a deflection coil mounted on the CRT to perform the horizontal scanning of the raster on the screen. The raster is located at the right end of the face plate at time point t.sub.2, and at the left end of the face plate at time point t.sub.4. The portion between time points t.sub.2 and t.sub.4 corresponds to the blanking period.
FIG. 8(b) shows a voltage waveform of a flyback pulse V.sub.f generated between the time points t.sub.2 and t.sub.4. In a conventional circuit, this waveform of the flyback pulse voltage V.sub.f is used to form a horizontal blanking pulse V.sub.BP as seen from FIG. 8(c).
Assume that this horizontal blanking pulse V.sub.BP is delayed by the time .DELTA.t as seen in FIG. 8(d). This delayed horizontal blanking pulse V.sub.BPD is able to erase the bright spot generated at the left end (time point t.sub.4) on the face plate, but not the bright spot generated at the right end (time t.sub.2) of the screen.